In order to realize a next-generation long-distance large-capacity communication system, research and development have been conducted regarding technology to generate a transmission signal using digital signal processing in an optical transmitter. For example, digital signal processing is used to generate a desired light signal waveform such as a dispersion pre-equalized signal and a modulation signal.
FIG. 1 is a diagram illustrating an example of an optical transmitter. The optical transmitter illustrated in FIG. 1 includes a light source (a laser diode (LD)) 11 and an optical modulator 12. The optical modulator 12 is, for example, a Mach-Zehnder-type lithium niobate (LN) modulator and includes an I arm and a Q arm. In addition, the optical modulator 12 includes a phase shifter to provide an optical phase difference of π/2 between the I arm and the Q arm.
Continuous (continuous wave (CW)) light generated by the light source 11 is split by an optical splitter and guided to the I arm and the Q arm of the optical modulator 12. In addition, a data signal I and a data signal Q are provided to the I arm and the Q arm, respectively, of the optical modulator 12. The amplitude of both the data signal I and the data signal Q is, for example, 2Vπ. Vπ is a voltage corresponding to a half cycle of a drive-voltage-to-light-intensity characteristic of an LN modulator (namely, a half-wave voltage). In the I arm, the continuous light is modulated with the data signal I to generate an I arm modulated light signal. Similarly, in the Q arm, the continuous light is modulated with the data signal Q to generate a Q arm modulated light signal. The I arm modulated light signal and the Q arm modulated light signal are combined to generate a QPSK modulated light signal.
In the optical transmitter including the above configuration, bias voltages for the I arm and the Q arm are appropriately controlled in order to generate a high-quality light signal. In order to control the bias voltage for the optical modulator 12, the optical transmitter includes a control section 13, a photodetector (PD) 14, and a detecting section 15.
The control section 13 superimposes a low-frequency signal on the bias voltage for the optical modulator 12. Hereinafter, f0 represents the frequency of the low-frequency signal. The modulated light signal output from the optical modulator 12 includes a frequency component (namely, the f0 component) of the low-frequency signal. The photodetector 14 converts the modulated light signal output from the optical modulator 12 into an electric signal. The detecting section 15 detects the intensity and phase of the f0 component included in the modulated light signal, based on the electric signal generated by the photodetector 14. The control section 13 performs feedback control on the bias voltages for the I arm and the Q arm so that the f0 component included in the modulated light signal approaches zero. As a result, the bias voltages for the I arm and the Q arm are optimized and a high-quality light signal may be generated. The above feedback control may be referred to as automatic bias control (ABC).
A method for controlling the bias of an optical modulator in an optical transmitter by using a low-frequency signal is disclosed in, for example, Japanese Laid-open Patent Application Publication No. 2000-162563.
The amplitude of a drive signal for the optical modulator (the data signal I and the data signal Q in FIG. 1) may change due to temperature or aging. However, the change of the amplitude of the drive signal due to temperature and aging is small. Thus, with an optical transmitter of the related art, the amplitude of the drive signal is almost uniform during operation of a communication system.
In contrast, in an optical transmitter that uses digital signal processing to generate a transmission signal, change of a modulation method and/or change of a pre-equalization amount may be performed during operation of a communication system. When the modulation method and/or the pre-equalization amount are changed, the amplitude of a drive signal for light modulation may change.
For example, FIG. 2A illustrates the waveform of a drive signal when the optical transmitter performs QPSK modulation, and FIG. 2B illustrates the waveform of a drive signal when the optical transmitter performs 16-QAM modulation. In this example, with QPSK modulation, the amplitude of the drive signal is about 2Vπ. In addition, with 16-QAM modulation, the amplitude of the drive signal is about 0.6Vπ. As described above, when the modulation method changes, the amplitude of the drive signal also changes.
FIG. 2C illustrates the waveform of a drive signal when the optical transmitter performs QPSK modulation and executes pre-equalization. In this case, the amplitude of the drive signal is smaller than Vπ. The pre-equalization is implemented by previously providing distortion to a signal waveform in the transmitter so as to compensate for chromatic dispersion of an optical transmission path between the transmitter and a receiver. In addition, the pre-equalization is implemented by digital signal processing.
As described above, in a recent or future optical transmitter, a drive condition of an optical modulator (the amplitude of the drive signal in the above example) may greatly change in response to change of the modulation method or the like. When the drive condition changes, the bias for the optical modulator may possibly not be appropriately controlled, and the optical transmitter may possibly not be able to generate a high-quality light signal. Hereinafter, a problem of bias control of the related art will be described with reference to the configuration illustrated in FIG. 1.
FIG. 3 is a diagram illustrating bias control when the drive amplitude Vd is greater than Vπ. This operating state corresponds to, for example, when a light signal is generated by the QPSK modulation illustrated in FIG. 2A. FIG. 4 is a diagram illustrating bias control when the drive amplitude Vd′ is less than Vπ. This operating state corresponds to, for example, the 16-QAM modulation illustrated in FIG. 2B or the pre-equalization illustrated in FIG. 2C.
In FIGS. 3 and 4, a bias voltage Vb for the optical modulator is shifted to the low-voltage side of an optimal point. In addition, a low-frequency signal f0 is superimposed on the bias voltage Vb.
When Vd is greater than Vπ, an f0 component A is generated at one edge of a drive signal and an f0 component B is generated at another edge of the drive signal as illustrated in FIG. 3. The f0 component A and the f0 component B are light signal components of the frequency f0 and are included in the modulated light signal. Therefore, the modulated light signal includes an f0 component C (C=A+B).
In this example, the amplitude of the f0 component B is higher than the amplitude of the f0 component A. Thus, the phase of the f0 component C is the same as the phase of the f0 component B. Here, the f0 component B is generated in a region in which the gradient of the drive-voltage-to-light-intensity characteristic is positive. Therefore, in this example, the phase of the f0 component C detected from the modulated light signal is the phase of the low-frequency signal superimposed on the bias voltage.
In this case, the bias voltage for the optical modulator is controlled based on the following rules (a) to (c).
(a) If the phase of the f0 component C is the same as the phase of the low-frequency signal superimposed on the bias voltage, the bias voltage is increased.
(b) If the phase of the f0 component C the opposite of the phase of the low-frequency signal superimposed on the bias voltage, the bias voltage is decreased.
(c) If the intensity of the f0 component C is zero, the bias voltage is maintained (zero includes a state of being less than a sufficiently low threshold).
In contrast, when Vd′ is less than Vπ as illustrated in FIG. 4, an f0 component A′ is generated at one edge of a drive signal and an f0 component B′ is generated at another edge of the drive signal. The f0 component A′ and the f0 component B′ are light signal components of the frequency f0 and are included in the modulated light signal. Thus, the modulated light signal includes an f0 component C′ (C′=A′+B′).
In this example, the amplitude of the f0 component A′ is higher than the amplitude of the f0 component B′. Therefore, the phase of the f0 component C′ is the same as the phase of the f0 component A′. Here, the f0 component A′ is generated in a region in which the gradient of the drive-voltage-to-light-intensity characteristic is negative. Thus, in this example, the phase of the f0 component C′ detected from the modulated light signal is the opposite of the phase of the low-frequency signal superimposed on the bias voltage.
In this case, if the optical transmitter 1 uses the rules (a) to (c), the bias voltage is not controlled so as to approach the optimal point. In other words, according to the rules (a) to (c), when the phase of the f0 component C′ is the opposite of the phase of the low-frequency signal superimposed on the bias voltage as illustrated in FIG. 4, control is performed so as to decrease the bias voltage. By so doing, the bias voltage is controlled in a direction in which a difference from the optimal point increases. As a result, feedback control for optimizing the bias voltage diverges.
As described above, in the related art, when the drive condition of the optical modulator changes, the bias voltage is not able to be appropriately controlled. When the bias voltage for the optical modulator is not appropriately controlled, the quality of a light signal transmitted from the optical transmitter deteriorates.